target/arm: Implement FEAT_HBC

FEAT_HBC (Hinted conditional branches) provides a new instruction
BC.cond, which behaves exactly like the existing B.cond except
that it provides a hint to the branch predictor about the
likely behaviour of the branch.

Since QEMU does not implement branch prediction, we can treat
this identically to B.cond.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

docs/system/arm/emulation.rst

@@ -42,6 +42,7 @@ the following architecture extensions:
 - FEAT_FlagM2 (Enhancements to flag manipulation instructions)
 - FEAT_GTG (Guest translation granule size)
 - FEAT_HAFDBS (Hardware management of the access flag and dirty bit state)
+- FEAT_HBC (Hinted conditional branches)
 - FEAT_HCX (Support for the HCRX_EL2 register)
 - FEAT_HPDS (Hierarchical permission disables)
 - FEAT_HPDS2 (Translation table page-based hardware attributes)